Lily is preparing two samples different
compounds so that each sample contains the
same mass of calcium. The first sample she
prepares is 10.0 grams of calcium phosphate,
Ca3(PO4)2. She is preparing her second sample
using calcium hydroxide, Ca(OH)2. What mass of
Ca(OH)2 should she prepare so that it contains
the same mass of calcium as the first sample?
Help?

3.84 grams is the mass of calcium used in 3 moles of Ca(OH)2 so that it contains  the same mass of calcium as the first sample.

Explanation:

Data given:

first sample:

10 grams of Ca3(PO4)2

second sample :

mass of Ca(OH)2 =?

mass of the calcium should be same in both the solutions

In Ca3(PO4)2, 3 moles of calcium is present so

mass = 3 x 40.07 ggrams/mole

mass of calcium= 120.21 grams in Ca3(PO4)2

In Ca(OH)2 only 1 mole of calcium is present so to equal the mass 3 moles should be taken

3 moles of Ca(OH)2 is taken

number of moles of calcium in grams of Ca3(PO4)2

= 0.032 moles.

1 mole of Ca3(PO4)2  has 3 moles of calcium

0.032 moles of  Ca3(PO4)2 has x moles

=

= 0.096 moles

so 0.096 moles or 3.84grams of calcium will be used to make solutions of

Ca3(PO4)2 and Ca(OH)2 separately of 10 grams each.